I'm trying to recall the name of a novel or short story that I read in mid-4th edition. The only details that I can recall at the moment are that the story involved a hag (a night hag, I believe) who was posing as a female clerk or attendant in some kind of shop, and that the shop owner was aware of the hag's identity.
Not much to go on, but I figured a hag posing as a beautiful shop clerk was odd enough that someone might remember it.
The OP's description also sounds somewhat like Ei-Vene the Embalmer, Old Mebbeth the Midwife, and Young Marta the Seamstress - a triad of "aspects", "fragments", or "parallel lives" of the ancient Hag, Ravel Puzzlewell. Each of these fine ladies offers the Nameless hero of Planescape: Torment their goods and services and favours ... for a price.